This invention relates to ceramic compositions having chemical stability and electronic conductivity at elevated temperatures. In particular, it relates to an all ceramic structure for a molten carbonate fuel cell and to compositions for use in other reducing and oxidizing environments at elevated temperatures.
Previous molten carbonate fuel cells have included an anode such as porous nickel, a cathode such as porous lithiated nickel oxide, a metallic interconnect, and molten carbonate as an electrolyte retained in a porous matrix. A representative electrolyte is a mixture of Li.sub.2 CO.sub.3 and K.sub.2 CO.sub.3. Typical operating temperatures of such molten temperatures are between 600.degree.-700.degree. C. and at such elevated temperatures there has been loss of physical stability of the anode structure over extended periods of operation, and corrosion of the interconnect.
Nickel metal as anode material is chemically, but not mechanically stable. A porous nickel anode will creep at the cell operating temperature and the required mechanical loading, resulting in reduced porosity. This problem has been addressed by alloying the nickel with chromium or aluminum, for creep resistance, but with accompanying increased cost and effort in anode production. Other efforts have involved plating ceramic particles with nickel to provide a hard core. Unfortunately, the nickel plating tends not to wet the ceramic and to coalesce into islands of nickel on otherwise uncoated particles.
Present designs of molten carbonate fuel cells including a nickel anode, a metal separator as an interconnect, a cathode with a dopant, and an electrolyte matrix of LiAlO.sub.2 ceramic, experience thermal expansion and sealing problems during fabrication, and corrosion of the interconnect during long-term operation. Performance improvement and increased range of viable applications will result from the use of a unitary material for all of the components, including wider temperature operating range.
The present inventors, jointly with Sim, have reported stable, electronically conductive ceramic cathode compositions in U.S. Pat. No. 4,564,567, which patent is incorporated herein by reference. These cathode compositions involved lithium-transition metal oxygenates with suitable dopants to increase their electrical conductivity to a level for molten carbonate cathode use. The inventors subsequently reported ceramic anode compositions in co-pending U.S. application Ser. No. 357,247 that are chemically stable in the molten alkali metal carbonate fuel cell. The anode compositions involve alkali metal-transition metal oxygenates in non-stoichiometric relation. Ceramics which are stable in both anode and cathode gases, and conductive in both environments, and the incorporation of a ceramic rather than a metallic separator sheet are herein disclosed. This disclosure incorporates the ceramic cathode compositions of U.S. Pat. No. 4,564,567, the ceramic anode compositions of copending U.S. application Serial No. 357,247, and further provides for a unitary, all-ceramic molten carbonate fuel cell.
Therefore, in view of the above, it is an object of the present invention to provide an electrically conductive ceramic structure of such compositions that are stable in the fuel gas and oxidizing environments.
It is a further object to provide a molten carbonate fuel cell with high power density in a nearly optimum temperature range and which has a wide range of applications.
Yet another object of the present invention is to provide a molten carbonate fuel cell that presents reduced sealing and expansion problems in fabrication.
Another object is to provide a molten carbonate fuel cell having improved corrosion resistance.